What it is
An insect that landed in wet mud at the edge of a pool, about 310 million years ago. It left the print of its whole body, an inch and a half long.
Why it matters
You can count six legs, the rings of its belly, even the dents of its claws. No tracks come in, and none go out. It did not walk there. It came in on wings. It touched down claws first, slid forward about a third of an inch, and settled. Then it left, and the next flood laid fine silt into the print.
Behind a strip mall
A geology student at Tufts University, Richard Knecht, found it in the woods behind a strip mall in North Attleboro, Massachusetts. He split the stone, and, in his words, opened it like a book: the landing was on both halves. He and two scientists described it in 2011.
Still argued
The team that described it thought it was an early cousin of the mayfly that flew down onto the mud. In 2013, another scientist argued it was more like a stonefly, an insect that skims across the water on its wings, and that it slid in from the surface. The two sides have answered each other, and the question is still open.